The present invention relates generally to the generation of unstable, i.e., radioactive, nuclear isotopes (often referred to as radioisotopes), and more particularly to techniques for generating medical isotopes such as molybdenum-99 (Mo-99) and its decay daughter technetium-99m (Tc-99m), and radioactive iodine such as iodine-131 (I-131).
Radioisotopes, in very small doses, are widely used in clinical therapy (radiation treatments) for such diseases as cancer and hyperthyroidism, as well as diagnostics using the ability to image regions where radioisotopes concentrate in the subject's body. Currently, nearly 80% of all nuclear imaging procedures utilize Tc-99m, making it a very important isotope for diagnostic medicine. Molecules and proteins that concentrate in specific areas of the body can be tagged with Tc-99m, which decays to a ground state through emission of a low energy gamma-ray, and observed from outside the body using gamma-ray cameras or detectors. This method allows “active” areas, or regions where the Tc-99m tagged compound concentrates, to be observed in 3-D from outside the body.
With a high demand for medical procedures involving the use of Tc-99m and I-131, a demand that is only expected to increase as the U.S. population ages, reliability of the Tc-99m and I-131 supplies is critical. A major obstacle to a reliable source is the fact that 100% of the U.S. supply is imported from foreign reactors. The U.S. supply is sourced almost entirely from the NRU Reactor in Canada (Chalk River) and the HFR in the Netherlands, and both reactors are over 40 years old. The rapid decay of the Mo-99 means that product must be shipped and used immediately with no long term storage possible. Any interruptions in supply, even brief periods such as a reactor shutting down for maintenance, can cause shortages and patient treatment delays. Real shortages have occurred as recently as 2007 and 2008 when the NRU Reactor and HFR, respectively, were shut down for a period of time.
Traditional methods rely on thermal fission of targets made of highly enriched uranium (HEU). HEU is uranium that has been processed to greatly increase the percentage of fissionable U-235 above the approximately 0.7% level, found in naturally occurring uranium, to levels above 93%. Thermal fission refers to the irradiation of a target by low-energy (“thermal”) neutrons, causing fission to occur. Currently, the U.S. exports more than 50 kg of HEU having more than 93% U-235 to at least five foreign nuclear reactors for irradiation and extraction of the Mo-99/Tc-99m and other medical isotopes. The proliferation potential and hazards associated with shipping fresh HEU and spent HEU are obvious, but current production of Mo-99/Tc-99m relies on this process.
The spent HEU target material is also a threat because only between I-3% of the U-235 in the HEU target is burned up and the remaining target material can still contain 92% enriched U-235. Also, because of the low burn-up, after three-year storage, the HEU target materials can be essentially contact handled, meaning that due to its relatively low burn up, the amount of long-lived fission products in the spent HEU target material are minimal. The spent HEU target can be handled and processed relatively easy with minimal shielding materials to protect the proliferators.
Alternative techniques have been proposed, but they are thought to be significantly less cost effective and many technical challenges remain. One such proposal is to transition to a lower level of enrichment of the U-235 target (LEU), say below 20% U-235, but this still presents the same problems as HEU, including the need for a nuclear reactor. Another method proposed is to utilize neutron capture in Mo-98, which can be mined as ore. However, natural molybdenum contains on the order of 24.1% Mo-98, so targets are likely to require enrichment prior to irradiation.
Other techniques proposed for production of Mo-99 include causing (p, 2n) or (p, pn) reactions in an Mo-100 target using a proton accelerator (cyclotron). Again, natural molybdenum contains on the order of 9.64% of Mo-100, so targets for cyclotrons are also likely to require enrichment prior to irradiation. It has also been proposed to cause (γ, f) reactions on U-235 or LEU, or U-238 or a combination of all three materials via bremsstrahlung radiation produced from a high-energy electron accelerator.
The proposed alternative methods using particle accelerators all have similar problems:                They all require large and enriched isotopic targets.        They all require heat removal from the targets during irradiation, which represents a technical challenge.        The Mo-99 produced must be purified to remove unused molybdenum isotopes and other fission products and activation by-products.        They all require development of fast dissolution methods for the metallic targets.        Treatment and disposal of the waste fission products and waste uranium present significant challenges (for LEU).        